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In Search of Perfect
Power:

Thoughts on the DoD's
"Wearable Power" Contest

Introduction

In the November 2007 issue of the ARRL's QST
Magazine, "Eclectic Technology" columnist Steve
Ford, WB8IMY, reported on the Department
of Defense's ongoing "Wearable Power" contest. The purpose
of this contest is to drive the development of a low-mass, high-density
energy source to power the ever-increasing number of electronic
gadgets our soldiers are expected to carry onto the battlefield.
The contest is open to contributions by the general public,
which means that this is a golden opportunity for America's best
basement engineers, garage inventors, and ham radio enthusiasts
to compete in an arena usually dominated by large defense contractors.
A first-prize winner takes home a million dollars. The runner-up
will have to "make-do" with a mere half-million.

Unfortunately, a payoff like that won't come
easily. To qualify for the prize, the winner must demonstrate an
electrical energy source capable of delivering an average of 20
watts (with peaks of up to 200 watts) for periods of up to
96 hours. At the same time, this energy source must weigh less than
4 kilograms (about 9 pounds). Finally, the mechanical proportions
and characteristics of the device must be suitable for attachment
to a vest or similar garment.

The idea behind the contest is an interesting
one and it caught my attention because ham radio operators, particularly
those interested in QRP, portable, or emergency communications,
are always on the lookout for new and better power sources. At
the same time, I would be lying if I claimed that a million dollar
prize wasn't equally inspiring. Either way, I decided that it was
worthwhile to do some Web surfing and a little bit of number-crunching
to get a sense of how difficult it might be to meet the contest's
design criterions.

How Much Energy
Are We Really Talking About?

To simplify things I decided set aside, for
the moment, the weight and peak power specifications and consider
the average power delivery requirement. If we multiply 20 watts
times 96 hours we get 1920 watt-hours. Energy may also be expressed
in the unit joules, and a unit conversion from watt-hours to joules
yields 6.9 million joules.

How does this quantity of energy relate to
the real world? If we divide 100 watts into 1920 watt-hours,
we can see that a winning energy source would be able to light a
100 watt light bulb for more than 19 hours. That's a lot of juice.

Another way to look at this is to consider
the energy required to lift something heavy. Lift energy is equal
to an object's mass, times the gravitational constant, times height.
Assuming a typical U.S. military Humvee weighs 5200 pounds, a winning
power source would be capable of providing sufficient electrical energy
to lift that vehicle to a height of almost 1000 feet. As you can
see, the DoD is not looking for a toy-- this is a real meat-and-potatoes
power source.

Bunches of Batteries

My next step was to see how this
energy requirement meshes with the delivery capability of common
power sources. Surfing the Web, I found an ELK-1280 lead-acid gel-cell
battery. This is the kind of battery commonly seen in computer
UPS's and alarm boxes. The battery is rated at 12 volts and 8 amp-hours,
a 96-watt-hour capacity. Divide this into the 1920-watt-hour requirement,
and we find that to build a contest-winning power pack from these
batteries, you'd need at least 20 of them connected in tandem. These
batteries weigh in at more than 6 pounds apiece. A full set of 20
weighs 123 pounds, or 56 kg. In short, a power pack based on these
cells fails the DoD weight requirement by a factor of 14, and we
haven't yet accounted for real-world factors that would drive the
need for additional batteries.

For example, the amp-hour rating of batteries
is based on a certain rate of discharge, and the relationship between
load and battery life is not linear. You might actually need more
cells (and therefore more weight) to assure sufficient power delivery
throughout the power pack's discharge cycle. Let's not forget that
battery capacity can be adversely affected by temperature extremes,
and note that we haven't even considered how 200-watt peak demands
might impact the design. No, lead-acid batteries are not the answer.

Granted, lead-acid technology is old-school,
so I surfed the Web some more, looking for data on nickel-cadmium
cells. I found a NiCad battery pack, part number PN-3600, with a
terminal voltage of 36 volts, and a capacity of 3.6 amp-hours. Multiply
these two numbers and divide the result into the 1920-watt-hour
requirement, and you discover that you need at least 15 of these
packs to meet the DoD specification. Unfortunately, the combined
weight of all the cells is almost 90 pounds, or 41 kilograms. Again,
we fail the weight requirement by a large factor.

Lithium-ion batteries are more "cutting-edge,"
but how do they stack up? On the web, I found a lithium-ion replacement
battery for the Ipod, part number IPOD-3G37V500. The pack is rated
at 3.6 volts, with a capacity of 0.5 amp-hours. The retail information
states the weight at 0.2 ounces. If you run the math, you need at
least 1037 batteries. The combined pack weighs in at nearly 13 pounds,
or 5.9 kilograms. The pack is one and one-half times too heavy. We're
getting closer to the goal, and yet, we are still so far away!

Very quickly, it becomes evident that any
common battery technology is not going work in this application.

Not a Plane, Not
a Bird... It's Super Capacitor!

Thirty years ago, when I began learning about
electronics, I remember reading about capacitors and their role
as energy storage devices. Universally, text from that era goes
on to explain that while the measurement unit of electrical capacity
is the farad, practical capacitors are always measured in microfarads
(millionths of a farad) or picofarads (trillionths of a farad.)
At the time, there was no such thing as a 1-farad capacitor.

How things have changed! So-called "ultra"
or "super" capacitors are starting to show up everywhere. Now, it
is not only possible to purchase a 1-farad capacitor, it's possible
to purchase capacitors rated at many hundreds of farads. There is
also a great deal of discussion as to whether these devices might
eventually represent real competition to traditional electrochemical
rechargeable batteries. I decided to investigate super-capacitors
in the context of the DoD's contest, and returned to the Web to
conduct some more research.

The results were at once intriguing and disappointing.
A visit to Maxwell Technology's Web site provided datasheets for
650-farad capacitors, from which I could calculate energy capacity
and weight. The model BCAP300-E270, for example, has an advertised
energy density of 5.52 watt-hours per kilogram. You'd need at least
348 kilograms worth to provide the energy required by the contest--
way too heavy.

A Wikipedia entry on the phrase "supercapacitor"
references a 2006 patent (number 7033406) filed by EEStor. According
to the article, energy densities as high as 342 watt-hours per kilogram
are claimed. This is most impressive, because it actually exceeds
the energy density that I had calculated for the lithium-ion battery,
and it shows that super-capacitor technology has a
bright future in rechargeable electronic devices. That said, a power
pack composed of EEStor capacitors still fails the contest's weigh
limits by a large amount.

Energy Density
and Liquid Fuels

A Google search on the phrase "energy density"
brought me to an excellent Wikipedia entry that lists energy density
values for a variety of fuels and energy sources. The energy density
values in this table are expressed as watt-hours per kilogram, so
direct comparisons are made easier if we translate the contest requirements
to similar units. We know a winning power pack design must supply
1920 watt-hours and weigh no more than 4 kilograms. Divide the second
figure into the first, and we get a target energy density of 480
watt-hours per kilogram. The point is this: In order for a contest
entry to be viable player, its energy density must meet or exceed 480 watt-hours
per kilogram.

Right off, we can dismiss certain "alternative"
ideas like using compressed air as a power source (only 34 watt-hours
per kilogram). Similarly, while significant research has gone into the idea of using
flywheels for energy storage, with an energy density
of only 120 watt-hours per kilogram, they don't make the cut.

What stands out in the Wikipedia table are
the hydrocarbon fuels. All of them exhibit energy densities that
are consistent with the demands imposed by the DoD's contest. The
question is how to perform the necessary chemical-to-electricity
conversion.

Combustion Engines

Burning fuel in an engine to drive a small
permanent magnet generator is an obvious approach, but as I see
it, it's not a very good one. Internal combustion engines are notoriously
inefficient. Even if we engineer our way past their weight, there
is a real problem with getting rid of the waste heat they produce.
A soldier wearing armor and NBC (nuclear/biological/chemical) gear
in the 112 degree heat of a place like Baghdad is not likely to
tolerate a "wearable" power source that blows hot exhaust gas against
his body. Engines also fall short in applications where stealth
is essential. Even muffled engine exhaust can be heard at great
distances, and through an infrared viewer, exhaust pipes may as
well be flashing beacons proclaiming, "Here I am! Shoot in this
direction!"

External combustion engines like low delta-T
Stirling motors may provide the answer, particularly the newer piston-less,
travelling-wave varieties. Pressure variations within these devices
have been used to drive piezo disks or linear alternators, resulting
in electricity production without rotating parts. However,
as in the case of internal combustion engines, efficiency, weight,
noise, and heat signature issues may render Stirling variants unsuitable
for application to "wearable" battlefield power sources.

Fuel Cells

It is possible to "burn" fuel without a flame,
and to harness that reaction to directly produce electricity using
an apparatus called a fuel cell. The principle behind the fuel cell
was recognized as early as the first half of the 19th century. Hydrogen
fuel cells have found use in U.S. spacecraft, where they provide
reliable power and even drinking water. In more recent years, research
efforts, both public and private, have sought to reduce the cost
of fuel cell technology, improve the efficiency and working-life
of the equipment, and to broaden the range of fuels that can be
consumed in the cell.

Revisiting Google and the Web, one of the
first fuel cell search "hits" I stumbled upon was the Web site of
UltraCell Products. Prominently featured is their model XX25 fuel
cell. The cell is capable of providing 25 watts of power continuously,
which meets the basic output specification as required by the contest.
The cell is fueled by a methanol/water mixture that is introduced
into the cell with preloaded cartridges. A single cartridge will
supply the cell with fuel for up to 9 hours, and cartridges are
"hot-swappable," so periodic replacement of empty cartridges with
fresh ones would allow the cell to produce energy almost indefinitely.

According to the manufacturer, the design
and packaging of the XX25 meets military standards for operation
under various humidity, temperature, dust, vibration, and shock
conditions, so practical implementation concerns have already been
addressed.

The XX25 itself weighs only 1.24 kilograms
(2.7 pounds), well below the target set by the contest. However,
if we add the weight of 10 or 11 fuel cartridges, enough fuel for
96 hours of operation, the total system weight crosses the 5-kilogram
mark. Another possible shortcoming of the XX25 is that there is
no indication on the product's datasheet that the
unit is capable of 200 watt surges. So, strictly speaking, the XX25
does not meet the contest criterions either.

Conclusions

Despite its shortcomings, the XX25 fuel cell
comes tantalizingly close to meeting the DoD's specifications. It
occurs to me that ethanol is similar to methanol, but contains 23%
more energy per unit weight. If it was possible to reconfigure the
cell to work with ethanol, it should be possible to produce
the same amount of electricity using less fuel or a smaller
fuel cell. This strategy might reduce the total system weight to
the desired 4-kilogram mark.

With regard to the contest's peak power requirement,
my thought would be to couple the fuel cell with a large super-capacitor
to give the system the ability to provide for demand surges.

It will be very interesting to see who wins
the contest and what technology is employed to capture the $1,000,000
prize. For what it's worth, I'd place my bet on fuel cell technology
or a fuel cell/ super-capacitor hybrid.

Information Sources

The following is a brief collection
of interesting URLs. Some of these provided the raw data used to
prepare the text above.

Department of Defense Research
and Engineering's "Wearable Power" Prize